alpha rays

Alpha rays \Alpha rays\ (Physics & Chem.)
   Rays of relatively low penetrating power emitted by radium
   and other radioactive substances, and shown to consist of
   positively charged {alpha particles} (helium nuclei) having
   enormous velocities but small masses. They are slightly
   deflected by a strong magnetic or electric field. Compare
   {beta ray} and {gamma ray}. Because the rays penetrate only a
   small distance in dense matter such as living tissue, small
   pellets of radioactive substances emitting alpha rays have
   been used to cause local destruction of tissue, as in cancer
   therapy.
   [Webster 1913 Suppl. +PJC]